I've already been given help on the different brand of filters, so I'm good there, but I do notice different sizes of filters, 55mm, 60mm, 77mm, etc, etc. How do I know which size will fit the camera? For those who don't know, I don't have the camera yet but I will be purchasing the Nikon D7100 and a 17-70mm sigma lens. Any help is greatly appreciated.

You will find the 17-70 Sigma lens info along with the lens literature,it would let you know the lens diameter and the size of the filter it takes.
Alternatively if you go to the camera store,people there would know what size filter it needs.

Okay, so I could have five different sized lenses and would need five different sized filters if I wanted a specific filter for all lenses? I'm assuming people only need one filter per camera and they use it primarily on one lenses. I'm also assuming that most people only use filters on prime lenses.

In general, filters like Neutral Density Filters or Circular Polarizing Filters would only be used for lenses that you might use to capture images whose composition contains a lot of sky, which would only include lenses with short focal lengths/wide angles of view. Also, those filters significantly limit the amount of light that gets through the lens, so you'd generally only use them with fast lenses. Thus, you wouldn't necessarily need a CPL or a ND for each and every lens.

Okay, I'm gonna hold off and just wait until I've had enough time with the camera and the photos I take...I may not even want one in the long run. I do appreciate everyone's input. I should probably be focusing more attention on a bag and possibly a tri-pod.

I had decided against getting any filters or any other lens accessories until I learned the camera. While doing a little research I came across the lens I was getting but it comes bundled with several filters and other things...all made by Sigma and all for $10-$20 more. I guess I'm wondering if it's worth a glance? Should I even consider it...even if I don't use them right away?